(1) Field of the Invention
This invention relates to a process for preparing an unsaturated nitrile/conjugated diene copolymer rubber wherein a monomer mixture comprising a conjugated diene monomer and a nitrile group-containing unsaturated monomer is polymerized in the state of being suspended in an aqueous reaction medium.
(2) Description of the Related Art
Several processes have heretofore been proposed for the commercial production of an unsaturated nitrile/conjugated diene copolymer rubber. A typical example of the proposed processes involves emulsifying a monomer mixture of a nitrile group-containing unsaturated monomer and a conjugated diene monomer in an aqueous medium by an appropriate emulsifying agent, and effecting the polymerization by incorporating a radical polymerization initiator in the monomer-emulsified aqueous reaction medium. The resulting copolymer has problems such that it contains a salient amount of residues including the emulsifying agent and an inorganic salt, exhibits a corrosive action for metals and has a poor electrically insulating property.
The above-mentioned problems can be solved to an appreciable extent by employing a solution polymerization process or a bulk polymerization process. However, the solution polymerization process results in a polymer having a low molecular weight, and the step of recovering a solvent from the polymerization mixture after polymerization is indispensable and the equipment cost is high. In the bulk polymerization process, the viscosity of the reaction mixture violently increases as the reaction proceeds, and an agitator of a high power is necessary, and it often becomes difficult to remove the heat of polymerization and control the polymerization.
A suspension polymerization process does not offer the above-mentioned problems of the solution or bulk polymerization process, but with regard to the preparation of an unsaturated nitrile/conjugated diene copolymer rubber, only one example is known wherein a monomer mixture is polymerized in an organic solvent (see Japanese Examined Patent Publication No. 50-33519). An attempt has not been made wherein an unsaturated nitrile/conjugated diene copolymer rubber is prepared by a suspension polymerization process using an aqueous medium, because blocking or sticking of polymer particles occurs to a considerable extent.